1. Field of the Invention
This invention relates to apparatus for controlling the flow of power in AC transmission lines, and more particularly to such apparatus providing control of transmission line impedance, transmission line voltage magnitude and/or transmission line voltage phase angle for unequal control of maximum power flow in the two directions in a transmission line including control of power flow in a single direction.
2. Background Information
A generalized power flow controller, also referred to as "unified power flow controller" (UPFC), is described in U.S. Pat. No. 5,343,139. This controller injects a voltage with controllable magnitude and angle in series with the transmission line in order to change the effective transmission line voltage, impedance and angle, and thereby control the real and reactive power flow in the line.
The UPFC described in U.S. Pat. No. 5,343,139 is able to both advance and retard the existing transmission angle by a "phase shift angle" .sigma. in the range -.sigma.max.ltoreq..sigma.+.sigma.max. Although this general capability of the UPFC is desirable in some practical applications, e.g., in those where power flow reversal is required, in many applications uni-directional change, advancement or retardation, of the transmission angle is satisfactory. In these applications, the solid-state inverters of the UPFC are not utilized well because with a given MVA rating they could control the transmission angle over the range of -.sigma.max.ltoreq..sigma..ltoreq.+.sigma.max, i.e., they could control up to the maximum angular change of 2.sigma.max, but they are actually used to control the transmission angle over either the positive range of 0.ltoreq..sigma.max or the negative range of .sigma.min.ltoreq.0, i.e., they are used to control up to a maximum angular change of only .sigma.max. Since the required MVA rating is proportional to the control range, it is easy to see that the solid-state inverters of the UPFC are only 50% utilized.
There is a need therefore an improved UPFC which is more efficient when applied to the control of power flow in transmission lines. More particularly, there is a need for an arrangement which allows full utilization of the UPFC over its total angle control range in those applications where only transmission angle advancement or retardation for power flow in a single direction is required, or where the maximum advancement and retardation angles for flow in the two directions are not equal.